We propose to study the following areas of Moloney strain of murine leukemia viruses (Mo-MuLV). 1. The role of MuLV gene products in replication and virus-cell interaction using temperature-sensitive and defective (non-conditional) mutants. 2. Interaction of ecotropic MuLV and endogenous xenotropic virus. A better understanding of the replication and organization of the leukemia viruses and how they interact with one another and with the host cells is necessary to evaluate the exact role of such viruses in cancer.